appetizer/food ideas for a spring cocktail party?

Avocado, mint and goat cheese spread. The only problem is the avocado getting brown (even if you spritz lemon it) - I guess you could ensure that the avocado is really covered in goat cheese
http://food52.com/recipes/17887-avocado-goat-cheese-mint-spread

Ramped-up Crostini with Ricotta and Pea Shoots
http://food52.com/recipes/4306-ramped-up-crostini-with-ricotta-and-pea-shoots

Asparagus and something with ham - as mentioned above, steamed asparagi wrapped in prosciutto

Smoky pickled asparagus spears. Nice serving idea of spears upright in glasses:
http://food52.com/recipes/4017-smoky-pickled-asparagus

Smoked trout, creme fraiche, dill on baguette toasts

Pea shooters - shot glasses of cold spring pea or pea shoot soup

Gougeres with ham or prosciutto bits and cheese. For 50, given the last-minute nature of the puffs, it would help to have a few slaves on hand. But nothing will disappear faster.

Baked brie and jam (apricot? that's springy) on toasts

Decor - if doing some of this outside, DIY ice luminary "candles"
Spring flowers or petals mixed with green leaves (azalea, cherry blossom, daffodil, grape hyacinth, etc.)
http://food52.com/blog/9545-diy-ice-luminaries
Or if ice won't work for your setting, line vases with leaves and flowers. Just do a search on the phrase "line vase with leaves" and you'll see plenty of tips

I love green soups in the spring, if you can serve in small glasses or tea cups. Sorrel soup, schav, watercress... hot or cold. I find martha Stewart's hors d'ouevre book inspiring and beautiful for this kind of menu. A toast with goat cheese and pea shoots or some other green... olives, always, other mild cheeses (more spring like), citrus (unless late enough for strawberries), maybe something lemony like small lemon tarts would be fresh tasting. Tiny lamb chops if you are serving meat eaters and have money to invest in the party. Vietnamese summer rolls can be made with guests if you have an adventurous crowd and are fresh tasting. I leave out the shrimp and pork, since they seem heavy to me. Devilled eggs are always delicious. Quail eggs are good with cocktails, can hard boil and serve with zaatar for dipping. They are so pretty. Carrots, raw, roasted, or grated in a salad. Chopped salad is easy to ea from a buffet since it is in tiny pieces.

Or you could serve salad in frico shaped like a taco. Of course you need asparagus some way, grilled or broiled, with or without prosciutto wrapping. Salmon mousse pipped onto cucumber rounds. Thinned goat cheese pipped into pea pods. Vietnamese Summer rolls. Savory biscotti would be make ahead. Something strawberry for dessert to complete the theme, or just a big bowl of berries and a bowl of sour cream and another of brown sugar for dipping. I recommend Martha Stewart Hors D'Oeuvres, it's got so many great ideas.

Whenever I have to cook for others, the theme is the first thing I decide upon. It simply helps to clarify my ideas and makes the search for food inspiration a lot easier. It's usually a cuisine - like Italian, Moroccan, Mexican or Thai. I think a single cuisine makes the food service overall more cohesive, and you can tailor your drinks to suit. Start with the lightest and freshest bites (cold), and end with the heaviest (warm), and or a 'dessert'.
Some easy-work items like a cheese plate, homemade bread sticks, nuts and dips reduce work, and give you time to prep items that need last minute attention. Even these mundane things can be dressed up nicely.
Oven roasted bite size 'meat balls' are easy to prepare - and can be made from fish, chicken, lamb, beef or pork, and flavoured to suit your chosen theme. Served with beautiful garnishes and sauces, they can impress more than the relevant amount of effort! A sticky sauce helps them colour in the oven, and an occassional shake helps them stay more round.
Roast marinated king shrimp (tails on) over salt, and serve on mini skewers.
A few lovely salads with some protein and crunch can be served on spoons to lighten things up.
Decorate serving plates with EDIBLE flowers, and freeze others in ice cubes for your drink pitchers.

really just a spring theme -- will have a ton of spring flowers around and then want a spring-appropriate menu (deviled eggs and whatever else I/we come up with...)

You mention you want the food to be "in line with theme," but not what that theme will be. We can be much more helpful to you if you clarify that aspect.